The present section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present disclosure that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Low-Dynamic-Range images (LDR images) are images whose luminance values are represented with a limited number of bits (most often 8 or 10). This limited representation does not allow correct rendering of small signal variations, in particular in dark and bright luminance ranges. In high-dynamic range images (HDR images), the signal representation is extended in order to maintain a high accuracy of the signal over its entire range. In HDR images, pixel values are usually represented in floating-point format (either 32-bit or 16-bit for each component, namely float or half-float), the most popular format being openEXR half-float format (16-bit per RGB component, i.e. 48 bits per pixel) or in integers with a long representation, typically at least 16 bits.
A typical approach for encoding an HDR image is to reduce the dynamic range of the image in order to encode the image by means of a traditional encoding scheme (initially configured to encode LDR images).
According to a first approach, a tone-mapping operator is applied to the input HDR image and the tone-mapped image is then encoded by means of a traditional 8-10 bit depth encoding scheme such as JPEG/JPEG200 or MPEG-2, H.264/AVC for video (Karsten Suhring, H.264/AVC Reference Software, http://iphome.hhi.de/suehring/tml/download/, the book of I. E. Richardson titled «H.264 and MPEG-4 video compression» published in J. Wiley & Sons in September 2003). Then, an inverse tone-mapping operator is applied to the decoded image and a residual is calculated between the input image and the decoded and inverse-tone-mapped image. Finally, the residual is encoded by means of a second traditional 8-10 bit-depth encoder scheme.
The main drawbacks of this first approach are the use of two encoding schemes and the limitation of the dynamic range of the input image to twice the dynamic range of a traditional encoding scheme (16-20 bits).
According to a second approach, the input HDR image is converted in order to get a visually lossless representation of the image pixels in a colour space in which values belong to a dynamic range which is compatible with a traditional 8-10 or an extended 12, 14 or 16 bits depth encoding scheme such as HEVC for example (B. Bross, W. J. Han, G. J. Sullivan, J. R. Ohm, T. Wiegand JCTVC-K1003, “High Efficiency Video Coding (HEVC) text specification draft 9,” October 2012) and its high bit-depth extensions.
Using a traditional encoding scheme for encoding an HDR image involves using a same bit depth for encoding the whole image. That involves large sizes of the compressed images. Consequently, such typical approaches cannot be used in applications requiring high coding performances such, for example, in a transmission context where high compression rates are required.